1. Field of the Invention
The present invention relates to a porous conducting metal oxide electrode prepared by depositing a porous conducting metal oxide film comprising a conducting metal oxide film layer having a network structure of nanofibers, comprising nanograins or nanoparticles, on at least one surface of a current collector, and a conducting metal oxide coating layer on the network layer of the porous conducting metal oxide through the constant current method or the cyclic voltammetric method, and a high-speed charge/discharge and ultrahigh-capacity supercapacitor using the porous conducting metal oxide electrode. The capacitor electrode, with a metal oxide film layer deposited on a porous conducting metal oxide film through an electrochemical method such as the constant current method or the cyclic voltammetric method, on a substrate in which a fine conducting metal oxide network layer is formed on a current collector, has ultrahigh-capacity and high-speed charge/discharge characteristics.
2. Description of the Background
Ruthenium oxide (RuO2), a conducting metal oxide, is a transition metal oxide having a tetragonal rutile structure. Especially, with good thermal and chemical resistance and superior electrical conductivity in spite of being an oxide, it has been widely used as an alternative for a metal electrode, a sensor, a catalyst, and the like.
Recently, researches have been actively carried out on the electrochemical wastewater treatment utilizing electrolytic oxidation of insoluble oxide catalyst electrodes as DSA (dimensionally stable anode). In particular, to improve the efficiency of pollutant oxidation, researches have been focused on the use of an anode prepared by forming a ruthenium-tin or iridium-tin oxide layer on titanium [Korean Patent Registration No. 10-0310272]. Further, researches on the application of electrochemical capacitors have been also carried out actively with the increased concerns on high-density, high-output energy storage systems.
A typical example is an electrochemical capacitor which stores charge through pseudocapacitance utilizing the reversible faradic surface redox reaction at the electrode/electrolyte interface.
Supercapacitors can be classified into active carbon-based, metal oxide-based pseudocapacitor based on reversible faradic surface redox reaction at the electrode/electrolyte interface, electrically conducting polymer-based capacitor enabling oxidation and reduction, and the like, depending on the electrode material.
Such an electrochemical capacitor may be used, alone or in combination with a secondary battery cell, as power source of compact medical appliances or mobile communication devices. It may also be utilized as power source of electric vehicles and hybrid cars. Examples of materials that exhibit pseudocapacitance include transition metal oxides such as IrO2 and RuO2. Until now, RuO2 has been known to provide the best characteristics for use as electrode for a supercapacitor.
To attain superior supercapacitor characteristics, the electrode material should have a large specific surface area and a low internal resistance. Also, the redox reaction at the surface in the available potential range should be continuous. In addition, researches on the utilization of the Pt—RuO2 electrode catalyst in direct-methanol fuel cells (DMFCs) are carried out actively. As such, transition metal oxides, particularly ruthenium oxide having superior electrical conductivity, can be utilized in various applications.
In this regard, it is important to prepare a ruthenium oxide having large specific surface area and porosity while maintaining superior electrical conductivity. For example, some researches on preparing ruthenium oxide having a nanowire structure using anodized aluminum oxide (AAO) as template in order to improve chemical reactivity by increasing surface area have been reported.
To be specific, Korean Patent Registration No. 10-1534845 discloses a preparation method of metal oxide electrode having a diameter in the range of from dozens to several hundreds of nanometers using AAO as a template. However when a template such as AAO is used, a good productivity cannot be attained. Further, since the size of the ruthenium oxide nanowire is restricted by the size of the template, this method is not suitable for large-scale production in view of cost competitiveness and reproducibility.
Accordingly, a process enabling a simple large-scale production of ruthenium oxide with a network structure of ultrafine nanograins and/or nanoparticles having a size of 5 to 30 nm is becoming more and more important. If it is possible to increase specific surface area of ruthenium oxide (RuO2) having superior electrical conductivity through such a simple process, superior characteristics can be attained in above-mentioned various applications, including electrode of supercapacitor, catalyst for DMFC, sensor, insoluble oxide catalyst electrode, conductive electrode, and the like. This can be attained with any conducting metal oxide having superior conductivity. That is, ruthenium oxide as well as conductive electrode materials having a conductivity of >0.1 S/cm. (IrOx, NiOx) may be utilized as electrode material for conductive electrodes and supercapacitors.
To this end, it is important to attain a nanofiber network structure of a metal oxide, including ruthenium oxide, IrOx and NiOx, to significantly increase specific surface area and realize a porous structure.
The inventors of the present invention have filed a patent application related to an ultra-sensitive metal oxide gas sensor and a preparation method thereof (Korean Patent Laid-open No. 2007-66859). In this invention, a metal oxide having a semiconductor characteristic is used to sense the change of resistance depending on gas adsorption. In the sensor, an oxide semiconductor having a band gap of approximately 3.2 to 4.5 eV, such as TiO2, SnO2, ZnO, etc., is used. The metal oxide semiconductor used in this invention is restricted in application as an electrode or electrode catalyst for replacing metal electrode or for ultrahigh-capacity, high-speed supercapacitors.